Well known in the sanitation, chemical, and petroleum industries is the separation of the phases of a fluid mixture by the sedimentation of particulant solids and the removal of oils lighter than water and undissolved gases by operations known as clarifying, decanting, settling, or simply, phase separating. In some more recent types of separation called here simply plate settling, the waste liquors containing solid particles flow through the horizontal lamellar spaces or channels between flat horizontal, parallel plates when assembled in a pack, as a pack of cards, each spaced at a slight distance from the next, longitudinal axis at an angle, and top and bottom edges parallel with the horizontal.
Relatively rapid settling or sedimentation of particulate solids as a sludge phase is accomplished while separating out a clear liquid phase, usually water. Or the same plate settling may be used simply for the decantating as a light liquid phase of oils, which term includes other liquids which are insoluble with and lighter than water. In other cases, both operations proceed simultaneously while undissolved gases may also rise to the surface; and the light phase may include a froth of light, flocculent solid particles which may have been entrained with sufficient bubbles of gases to make them float, while denser solids are concentrated at the bottom and removed as the sludge phase.
The term "phase" as used here is not applied in the true chemical sense; instead it means a "layer" of material having a more or less definite interface with another layer. Thus "aqueous" or "water" phase may be used to mean the relatively particle free and droplet free bulk layer principally of the carrying water--usually a solution of soluble materials; "sludge phase" may mean the heavier solid-slurry of particles; and the "oil" or "froth" phase may mean a phase with an apparent density less than that of the water phase, such as the lighter mixture or suspension of oils, lighter solids, and entrained gas bubbles or indeed gases alone; and it may be called simply the "light" phase, particularly if it is partly or entirely gaseous. A "fluid" mixture is one which may contain in addition to a liquid--usually water--other liquids and undissolved gases, also suspended solids but not in such large amounts as would prevent flow of the mixture in channels.
Previously described also has been the Pressure-Recycling Oxidation Sewage Treatment which includes the sedimentation or settling out of solid particles as a sludge simultaneously with the circulation of an influent of waste liquors as sewage to which air or oxygen is being supplied to satisfy a chemical oxygen demand (COD) and a biological oxygen demand (BOD). After preliminary screening and comminution of solids, the cycle may involve--first, a pressurizing with an oxygen containing gas to obtain a much greater solubility, concentration and activity of oxygen at the high pressures, compared with those at atmospheric pressure; second, an oxidation of organic components--often with a simultaneous sedimentation of insoluble particulate solids as a sludge; third, a depressurization with a release of dissolved gasses; fourth a recycling back to the influent of some part of the treated liquor and/or the sludge if formed and separated; and fifth, a withdrawal of a more pure effluent. This treatment is described in U.S. Pat. Nos. 3,772,187 and 3,788,476.